Structural Changes of the Prion Protein in Lipid Membranes Leading to Aggregation and Fibrillization

Abstract

Prion diseases are associated with a major refolding event of the normal cellular prion protein, PrP(C), where the predominantly alpha-helical and random coil structure of PrP(C) is converted into a beta-sheet-rich aggregated form, PrP(Sc). Under normal physiological conditions PrP(C) is attached to the outer leaflet of the plasma membrane via a GPI anchor, and it is plausible that an interaction between PrP and lipid membranes could be involved in the conversion of PrP(C) into PrP(Sc). Recombinant PrP can be refolded into an alpha-helical structure, designated alpha-PrP isoform, or into beta-sheet-rich states, designated beta-PrP isoform. The current study investigates the binding of beta-PrP to model lipid membranes and compares the structural changes in alpha- and beta-PrP induced upon membrane binding. beta-PrP binds to negatively charged POPG membranes and to raft membranes composed of DPPC, cholesterol, and sphingomyelin. Binding of beta-PrP to raft membranes results in substantial unfolding of beta-PrP. This membrane-associated largely unfolded state of PrP is slowly converted into fibrils. In contrast, beta-PrP and alpha-PrP gain structure with POPG membranes, which instead leads to amorphous aggregates. Furthermore, binding of beta-PrP to POPG has a disruptive effect on the integrity of the lipid bilayer, leading to total release of vesicle contents, whereas raft vesicles are not destabilized upon binding of beta-PrP.

Formation of native prions from minimal components in vitro

“…Further studies are required to distinguish between these possibilities and to identify the specific endogenous polyanions that facilitate prion propagation in vivo. Several classes of negatively charged macromolecules could potentially serve as cofactors for prion propagation, including: nucleic acids (17,(32)(33)(34)(35)(36)(37)(38), glycosaminoglycans (14-16), phospholipid-rich membranes (39)(40)(41)(42), and chaperone proteins (43). Interestingly, it has been proposed that polyanions could play a broad role in protein folding and misfolding, and the ability of polyanions to facilitate prion conversion may represent a specific example of that general concept (44).…”

Misfolding of the prion protein: linking biophysical and biological approaches

“…Although these two recombinant forms are neither glycosilated nor GPI anchored, they undergo a structural transition from an α-helical state to a β-sheeted structure that mimick on a conformational point of view the PrP C to PrP Sc conversion. The interaction of α-helical recombinant PrP with model lipid membranes has been characterized [75,107] as well as those of the β-sheeted form [24,55].…”

“…Both native and converted forms bind to negatively charged lipid membranes in a pH-dependent manner [24,55,75,107]. No binding occurs when prion protein interacts with pure zwitterionic phosphatidylcholine (PC) lipids.…”

“…No binding occurs when prion protein interacts with pure zwitterionic phosphatidylcholine (PC) lipids. But both native and β-sheeted conformers bind to model raft membranes constituted of PC, cholesterol and sphingomyelin [55,107]. The native conformer interacts with rafts only at pH 7, while the β-sheeted form binds strongly at both neutral and acidic pH [24].…”

“…Both native and β-sheeted forms of the N-truncated PrP gain in β-structure upon binding to negatively charged phosphatidylglycerol (PG) vesicles leading to total release of the lipid vesicles [55,107]. These interactions with negatively charged vesicles result in PrP aggregation whatever the initial conformation of the PrP in any pH studied.…”